1. Field of the Invention
The present invention relates to an organic electro-luminescent display device, and more particularly, to an organic electro-luminescent display device and a method for driving the organic electro-luminescent display device.
2. Discussion of the Related Art
Various flat panel display devices have been developed to reduce weight and volume which are disadvantages of a cathode ray tube. These flat panel display devices may be, for example, a liquid crystal display, a field emission display, a plasma display panel, an electro-luminescent display, and the like.
Research has been actively done for increasing the display quality and screen of such flat panel display devices. The electro-luminescent display, among them, is a spontaneous emission device that emits light by itself. This electro-luminescent display displays a video image by electrically exciting fluorescent material using carriers such as electrons and holes. Such electro-luminescent displays are roughly classified into an inorganic electro-luminescent display device and an organic electro-luminescent display device according to the type of materials used therein. The organic electro-luminescent display device is driven at a low voltage of about 5 to 20V. The organic electro-luminescent display device can be driven at a direct current (DC) low voltage as compared with the inorganic electro-luminescent display device which requires a high drive voltage of 100 to 200V. The organic electro-luminescent display device also has superior characteristics of a wide viewing angle, a high-speed response, a high contrast ratio, etc., so that it can be utilized as a pixel of a graphic display, or a pixel of a television image display or surface light source. In addition, because the organic electro-luminescent display device is thin, light and colorful, it is suitable as a next-generation flat panel display.
On the other hand, a passive matrix type driving system having no separate thin film transistor is mainly used as a driving system of the organic electro-luminescent display device.
However, the passive matrix type driving system has many limiting factors in resolution, power consumption, lifetime, etc. For this reason, efforts have recently been made to research and develop an active matrix type electro-luminescent display device for fabrication of a next-generation display requiring a high resolution or large screen.
FIG. 1 is a circuit diagram showing a basic pixel structure of a conventional active matrix type organic electro-luminescent display device.
The basic pixel structure of the conventional active matrix type organic electro-luminescent display device includes, as shown in FIG. 1, a gate line GL arranged in one direction, a data line DL arranged perpendicularly to the gate line GL, an electro-luminescent element D formed in a pixel defined by the gate line GL and the data line DL, a voltage supply line 110 for supplying a DC voltage to the anode of the electro-luminescent element D, a first NMOS transistor NT1 having a gate terminal connected to the gate line GL and a drain terminal connected to the data line DL, a second NMOS transistor NT2 having a gate terminal connected to the source terminal of the first NMOS transistor NT1, a drain terminal connected to the cathode of the electro-luminescent element D and a source terminal connected to a ground terminal, and a capacitor Cst connected between the gate terminal and source terminal of the second NMOS transistor NT2.
The first NMOS transistor NT1 is turned on in response to a scan signal from the gate line GL to form a current path between the source terminal and drain terminal thereof. The first NMOS transistor NT1 is also turned off when the voltage on the gate line GL is lower than a threshold voltage Vth thereof. During a turn-on time of the first NMOS transistor NT1, a data voltage from the data line DL is applied to the gate terminal of the second NMOS transistor NT2 through the drain terminal of the first NMOS transistor NT1. When the first NMOS transistor NT1 is turned off, the current path between the source terminal and drain terminal of the first NMOS transistor NT1 is opened, thereby causing the data voltage not to be applied to the gate terminal of the second NMOS transistor NT2.
The second NMOS transistor NT2 adjusts the amount of current flowing between the source terminal and drain terminal thereof according to the level of the data voltage applied to the gate terminal thereof to actuate the electro-luminescent element D so as to emit light of an intensity corresponding to the data voltage.
The capacitor Cst maintains the data voltage applied to the gate terminal of the second NMOS transistor NT2 constantly for a period of one frame. The capacitor Cst also maintains current applied to the electro-luminescent element D constantly for the period of one frame.
Meanwhile, the data voltage applied to the gate terminal of the second NMOS transistor NT2 always has a constant polarity (positive polarity), and the source terminal of the second NMOS transistor NT2 is connected to the ground terminal. As a result, the gate-source voltage of the second NMOS transistor NT2 always has the positive polarity, resulting in a problem in that the threshold voltage of the second NMOS transistor NT2 rises continuously toward one polarity (positive polarity). The rising of the threshold voltage of the second NMOS transistor NT2 causes a reduction in the amount of current supplied to the electro-luminescent element D and, in turn, a reduction in brightness of the electro-luminescent element D, which leads to a degradation in image quality. Therefore, a need exists for providing a driver for the electro-luminescent element that eliminates the degradation in image quality due to the continuous rise in the threshold voltage.